Laser light has been used for a variety of applications in recent years, for example, it performs metal cutting and processing, it is used as the light source of photolithography apparatuses in semiconductor fabrication apparatuses, it is used in various measuring apparatuses, and it is used in medical operation and treatment apparatuses such as in surgery, ophthalmology and dentistry.
In the case in which a solid state laser (in the present specification, a semiconductor laser (used as a concept that includes diode lasers)) is used as such a laser light source, the wavelength of the laser light that is emitted from the solid state laser is from the visible range to the infrared range, and a method of directly generating ultraviolet light has not been established. For example, it is not suitable for use in inspecting apparatuses, since the wavelength is too long. Therefore, a method of using such long wavelength light emitted from a solid state laser by converting it into a deep ultraviolet light of a short wavelength (for example, an eighth harmonic: wavelength of 193 nm) by using nonlinear optical crystals has been developed, for example, as described in Japanese Patent Application Publication No. 2001-353176 (Patent Document 1). BBO crystals, LBO crystals and CLBO crystals are known nonlinear optical crystals used for such a purpose.
In an such a laser light source, for example, in general, laser light generated from a DFB-LD is amplified using a plurality of optical fiber amplifiers (FDFA) and is then made into deep ultraviolet light by means of a wavelength conversion optical system such as the above.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-353176